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Short Communication
Description of a new natural Sonneratia hybrid from Hainan Island, China
expand article infoCairong Zhong§, Donglin Li|, Ying Zhang§
‡ Hainan Academy of Forestry, Hainan Mangrove Research Institute, Haikou, China
§ Lingnan Normal University, Zhanjiang, China
| Shaoguan University, Shaoguan, China
Open Access

Abstract

Here, we describe, illustrate and compare a new natural hybrid, Sonneratia × zhongcairongii Y. S. Wang & S. H. Shi (Sonneratiaceae), with its possible parent species. Based on its morphological characteristics and habitat conditions, this taxon is considered to represent a sterile hybrid between S. alba and S. apetala. In China, the new hybrid is only reported in the mangrove forest in Dongzhai Harbour, Hainan Island. It has intermediate characteristics with its parents by elliptical leaf blades, peltate stigma, terminal or axillary inflorescence with 1–3 flower dichasia, cup – shaped calyx (4–6 calyx lobes) and no petals. We also provide a key for the identification of Sonneratia species.

Keywords

Sonneratia, new hybrid, Dongzhai Harbour, Hainan Island

Introduction

Sonneratiaceae is a small tropical plant family consisting of only two small genera, Sonneratia and Duabanga. The inland genus Duabanga is an evergreen component of the rainforest belt, comprising two species within a more restricted range in Malaysian, Indonesia and China (Tomlinson 1986; Goutham-Bharathi et al. 2012). Sonneratia, a genus of mangroves, is one of the most important components of the intertidal zones of the tropical and subtropical coastal regions, ranging from East Africa through Indo-Malaya to tropical Australia and into Micronesia and Melanesia (Tomlinson 1986). This genus is also well-adapted to harsh intertidal zones with high salinity, hypoxia and ultraviolet (UV) radiation (Duke et al. 1998).

Sonneratia consists of six species and three interspecific hybrids (Duke and Jackes 1987; Duke 1994; Goutham-Bharathi et al. 2012; Yang et al. 2016). Amongst these, S. alba, S. caseolaris, S. ovata and S. × gulngai are the most widespread species (Tomlinson 1986; Goutham-Bharathi et al. 2012; Yang et al. 2016), whereas S. lanceolata and S. x urama are strictly limited to north-western Australia, southern New Guinea and a few locations in Indonesia (Yang et al. 2016). S. griffithii has a restricted distribution along the shores of the Andaman Sea, north to Bengal and south to the upper Malay Peninsula (Tomlinson 1986). S. x hainanensis, a hybrid derived from the cross between S. alba and S. ovata, is found in Hainan, China (Ko 1985; Wang et al. 1999). It was first reported that S. x hainanensis was in north-western Borneo as a nom. nud., based on morphological and cytological analyses (Muller and Hou-Liu 1966; Zhou et al. 2005). The parents of S. x hainanensis are widely distributed; however, more collections are needed. The mangrove S. apetala is restricted to southern India and Burma and is the most distinctive species because of its mushroom-like stigma (Tomlinson 1986). In China, S. apetala was first introduced in Dongzai Harbour, Hainan Island in 1985 from Bangladesh (Peng et al. 2012). Due to its accelerated growth and high tolerance of environmental stresses, S. apetala has been used as the pioneer species for mangrove restoration in estuarine and coastal areas. The species, S. alba, is an endemic species on Hainan Island (Li et al. 2017). Between two mixed populations, plants with intermediate characteristics have recently been encountered that obviously belong to the hybrid. In this study, we describe the new hybrid, S. x zhongcairongii and its features that distinguish it from both parent species.

Materials and method

The morphology of Sonneratia species presented here is based on field, vegetative and reproductive characteristics. Field traits were recorded on site, whereas vegetative and reproductive characteristics were observed and measured using fresh specimens, material preserved in 70% ethanol or press-dried specimens. Digital calipers (Mitutoyo, Japan) and a dissecting microscope with calibrated eye (Olympus, Germany) were used to describe the detailed morphological characteristics of samples. All photographs were taken in the field, i.e. in the natural habitat of the species, using a digital camera (cannon EOS RP, Japan). The morphological characteristics of Sonneratia species in Hainan Island have been summarized in a key to facilitate identification.

Results

The results of morphometric analysis showed that S. x zhongcairongii is more similar to its parents S. alba and S. apetala than to other Sonneratia taxa (Fig. 1, Table 1). Additionally, the morphology analysis of S. x zhongcairongii was intermediate between that of its parents (Figs 2, 3). The flowers of S. x zhongcairongii contained several abnormally-developed anthers (Fig. 1L), which might account for the 100% abortion rate and consequently the lack of fruit and seed (Table 1).

Table 1.

Comparison of described Sonneratia taxa and the new Hainan material.

Character S. × zhongcairongii S. alba [1-2] S. apetala [3] S. caseolaris [1-2] S. × gulngai [1-2] S. lanceolata [1-2] S. ovata [1,3] S. × hainanensis [4] S. griffithii [3,5]
Leaf blades elliptic obovate or elliptic to ovate narrowly elliptic to lanceolate elliptic elliptic elliptic broadly ovate elliptic or broadly elliptic obovate or suborbicular
Leaf apices rounded mucronate rounded rounded mucronate apiculate, mucronate apiculate, mucronate apiculate, mucronate rounded rounded obovate mucronate
Leaf base attenuate oblique attenuate oblique attenuate oblique attenuate oblique attenuate oblique attenuate oblique reniform broadly cuneate cuneate
Peduncle terete terete terete terete or tetragonous terete terete or tetragonous terete terete terete
Calyx lobes 4~6; inner often green 6~7(8); inner often red 4; inner often green 5~7; inner often red-streaked 5~7; inner often green 5~7; inner rarely red-streaked 6; inner often red at base 6; inner often red 6-7: inner often green
Petals absent white[6], linear-spathulate absent red, linear red, linear red, linear, rarely double absent white white (absent)*
Stamen white white white red, rarely white red white white white white
Stigma Mushroom-like, to 5~7 mm wide capitate but not expanded, 1~3 mm wide Mushroom-like, to 7~10 mm wide capitate but not expanded, to 3 mm wide capitate but not expanded, to 1.7 mm wide capitate but not expanded, to 3 mm wide capitate but not expanded, to 3 mm wide capitate but not expanded, to 3 mm wide capitate but not expanded, to 3 mm wide
Inflorescence terminal or axillary, 1-3(-5)-flowered dichasia terminal cyme occur either solitarily or in groups of three terminal cyme from branch axis terminal or axillary, 1-3(-5)-flowered dichasia terminal or axillary, 1-3-flowered dichasia terminal or axillary,1(-2)-flowered dichasia terminal cyme or solitarily or in groups of three terminal cyme 1-3(-5)-flowered dichasia terminal cyme 1(-2)-flowered dichasia
calyx (hypanthium) cup-shaped cup-shaped flat-expanded flat-expanded cup-shaped flat-expanded cup-expanded cup-shaped cup-shaped
Fruit Not developed Width = corolla width Width = corolla width width 5 mm > corolla width Width = corolla width width 5 mm > corolla width width 6-8 mm > corolla width width 5 mm > corolla width Width = corolla width
Seeds Not developed falcate falcate angular irregular angular irregular angular irregular rounded irregular angular irregular angular
Figure 1. 

Morphology of S. × zhongcairongii. A Habitat B stem with aerial root C bark D branches E leaf branch end with flowers F leaves G inflorescence H minute bract at a dichotomous inflorescence branch I flower J–L. Dissection of the flower (J), pistil (K) and stamens (L).

Taxonomic treatment

S. × zhongcairongii Y. S. Wang & S. H. Shi, nothosp. nov.

Figure 1

Material

Dongzhai Harbour, mangrove forest Hainan Island, China (Fig. 1A), 19°58'12"N, 110°34'48"E, 13 June 2018, Cairong Zhong, No. Saa20180613-001 (Holotype: IBSC; Isotype: IBSC).

Morphological traits

Trees, evergreen, 10–12 m tall, highly branched (Fig. 2A). Bark smooth or lightly fissured flaky, dark grey to pale fleshy green; stem base simple. Leaves simple, opposite, leaf blade leathery, glabrous, pale green, elliptical, 2–9 cm long,1–5 cm wide, apex obtuse, base acuminate, margin entire; petiole 0.3–1 cm; stipules absent. Inflorescence terminal or axillary, 1–3 or 1–5 flowered dichasia; flower bud ellipsoidal, 1.5–2.4 cm in length, 1–1.5 cm width, constricted medially, green, glossy, smooth, slightly angular; Calyx cup-shaped, lobes 4–6, wide ovate (0.8–1.2 cm long, 0.5–0.8 cm wide), apex acute, inner often fleshy green inside. Petals absent; stamens numerous along calyx, white, 1–1.5 cm in length; stigma peltate to 5 mm wide. Fruits not developed.

Figure 2. 

Comparison of the three taxa A S. apetala B S. × zhongcairongii C S. alba 1 leaves, 2 branches, 3 flowers.

Distribution

The hybrid is currently found only in Dongzhai Harbour within an area of 48 m2, mangrove forest, Hainan Island, China.

Habitat and ecology

The hybrid grows in a mangrove forest.

Phenology

The new hybrid flowered from the beginning of March to the end of October.

Conservation status

The new hybrid S. × zhongcairongii was collected only from the mangrove forest in Dongzhai Harbour. At this site, only two individuals were observed.

Discussion

To date, only three hybrids including S. × zhongcairongii have been reported in the genus Sonneratia. As with S. × zhongcairongii, other two hybrids have restricted location in the cross distribution of each parents (Duke and Jackes 1987; Duke 1994; Goutham-Bharathi et al. 2012; Yang et al. 2016). Only two individuals of the new hybrid were observed in China. The parent, S. apetala, is an exotic species in China, whose mixed location with S. alba started from 1985 (Peng et al. 2012). The morphological characteristics of S. × zhongcairongii were intermediate between its parents (Figs 2, 3); this result is consistent with the other two Sonneratia hybrids (Tomlinson 1986; Duke and Jackes 1987). S. × zhongcairongii showed complete abortion. However, on the other two hybrids (S. × gulngai and S. × hainanensis) can be found fruit and seeds with heavy abortion degrees (Tomlinson 1986; Wang and Wang 2007).

Figure 3. 

Schematic diagrams of Sonneratia taxa and their inter-specific affinities deduced from morphometric analyses. The choice of circle size and line length is arbitrary.

Backer and van Steenis (1951) compiled a thorough review of the Sonneratiaceae, a family of the order Myrtales. Two genera were described and include Duabanga and sonneratia. Gao Yunzhang divided the Sonneratia genus into two sections, sect. Sonneratia and sect. Pseudosonneratia, based on the presence or absence of petals (Ko 1985) and which was also used in the research of sonneratia Linn. in Australia, New Guinea and the south-western Pacific region (Backer and van Steenis 1951). By adding one new species found in China (S. paracaseolaris Ko, E. Y. Chen et W. Y. Chen), Gao Yunzhang regrouped the Sonneratia species in China (Ko 1993). Subsequently, a detailed anatomical analysis containing morphology of leaf, flower, fruit, seed and wood of five species of Sonneratia Linn. in China showed that the use of petal presence or absence was appropriate to distinguish species in Sonneratia Linn. (Chen 1996). Duke and Jackes worried about the use of petal presence or absence to distinguish between apetalous S. alba with S. ovata which was found to be less common, normally apetalous (Duke and Jackes 1987). Then the wrong character of S. alba was revised from apetalous to white, linear-spathulate (Wang and Wang 2007). Compared with characters of petal, stamen, leaf and flower bud, the stigma morphological characteristics have been used to group nine species and hybrids in Sonneratia Linn. (Wang and Chen 2002). In this study, we combined the use of petal presence or absence and stigma morphological characteristics to regroup Sonneratia plants and the new hybrid was most closely related to one of its parents, S. apetala.

To better distinguish amongst species belonging to the genus Sonneratia, we created a classification as shown in Table 1. The distribution range of the hybrid S. × zhongcairongii often overlaps with that of S. alba Smith. and S. apetala Buch. -Ham., which provides the possibility of formation of the hybrid species. The same is true for S. × gulngai N. C. Duke, S. × hainanensis Ko, E. Y. Chen et W. Y. Chen (Wang et al. 1999). The overlapping distributions of parent species contributed to the greater opportunity to form a natural hybrid (Zhou et al. 2008). Interestingly, one of the parents of all three hybrids is S. alba, which may be due to the fact that S. alba is a widely-distributed species, although further investigation is needed to determine the exact reason.

Key for the classification of Sonneratia species in China

1 Petals present 2
Petals absent 3
2 Petals white 4
Petals red 5
3 Stigma capitate but not expanded 6
Stigma mushroom-like 7
4 Leaf blades obovate or elliptical to ovate S. alba
Leaf blades elliptic or broadly elliptical S. × hainanensis
5 Fruit calyx flat-expanded, fruit width > corolla width by 5 mm 8
Fruit calyx cup-shaped, Width = corolla width S. × gulngai
6 Leaf blade apices rounded S. ovata
Leaf blade apices obovate mucronate S. griffithii
7 Flat-expanded calyx, fruit present S. apetala
Cup-shaped calyx, fruit absent S. × zhongcairongii
8 Leaf blade apices rounded S. caseolaris
Leaf blade apices apiculate, mucronate S. lanceolata

Acknowledgements

This study was supported by National Science Foundation of Hainan Province, China (grant numbers 318MS176; 319QN214); the National Natural Science Foundation of China (grant number 41776148).

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